Concept: Laetoli

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Laetoli is a well-known palaeontological locality in northern Tanzania whose outstanding record includes the earliest hominin footprints in the world (3.66 million years old), discovered in 1978 at Site G and attributed to Australopithecus afarensis. Here, we report hominin tracks unearthed in the new Site S at Laetoli and referred to two bipedal individuals (S1 and S2) moving on the same palaeosurface and in the same direction as the three hominins documented at Site G. The stature estimates for S1 greatly exceed those previously reconstructed for Au. afarensis from both skeletal material and footprint data. In combination with a comparative reappraisal of the Site G footprints, the evidence collected here embodies very important additions to the Pliocene record of hominin behaviour and morphology. Our results are consistent with considerable body size variation and, probably, degree of sexual dimorphism within a single species of bipedal hominins as early as 3.66 million years ago.

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The longitudinal arch is a unique characteristic of the human foot, yet the timing and pattern of its evolution remain controversial, in part due to the disagreement among researchers over which skeletal traits are the best indicators of its presence or absence. The small size of the human navicular tuberosity has previously been linked to the presence of a longitudinal arch, implying that the large tuberosity of early hominins such as Australopithecus afarensis reflects a flat foot. However, this hypothesis is at odds with other evidence of pedal form and function, such as metatarsal, tarsal, and footprint morphology, which show that a longitudinal arch was probably present in A. afarensis. This study reevaluates the morphometric affinities of the A. afarensis naviculars among other Plio-Pleistocene fossil hominins and anthropoid primates (N = 170). Multivariate cluster analyses show that all fossil hominin naviculars, including those attributed to A. afarensis, are most similar to modern humans. A measure of navicular tuberosity size quantified as the ratio of the tuberosity volume to the surface area of the talar facet shows that Ateles has the largest navicular tuberosity among the anthropoid sample and that there is no difference between highly arboreal and terrestrial taxa in this metric (e.g., Hylobates and Gorilla beringei). Instead, a relatively large navicular tuberosity may reflect the development of leg musculature associated with ankle plantarflexion. The functional inferences derived from the morphology of the A. afarensis naviculars are consistent with the morphology of the Laetoli footprints.